1. Field of the Invention
The present invention relates generally to the field of X-ray optics and more specifically to an improved synthetic multilayer collimator useful for X-ray lithography in modern semiconductor processing.
2. Prior Art
The following are United States patents that are related to the present invention. U.S. Pat. No. 4,200,395 teaches a method for precision alignment of two layers for X-ray lithography. U.S. Pat. No. 3,743,842 teaches an apparatus and method for X-ray lithography using an electron gun as the X-ray source. U.S. Pat. No. 3,984,680 teaches an apparatus for alignment of two layers in a soft X-ray lithography system.
X-ray lithography requires an illumination source whose wavelength, .lambda., is between 4 and 20 A. The source must be intense, small in extent (as viewed from the resist plane), of low angular divergence, well collimated, and must produce a uniform resist response. Synchrotrons satisfy all of these requirements, but researchers and manufacturers alike have sought to find physically more compact alternatives. These alternatives are generally called point sources, and the best candidate among them is the laser produced plasma source (LPP).
A problem common to all point sources is that they radiate nearly isotropically. Consequently, satisfactory collimation, or making parallel, is usually achieved by placing the lithography mask relatively far away from the point source, or making the mask field small. In other words, the mask must subtend a small solid angle, .OMEGA., leaving most of the isotropically generated radiation unavailable for productive use. This is undesirable due to the high cost of generating X-rays using a LPP source.
Collimating the isotropic X-rays from the LPP source allows better control of the pattern to be defined by lithography and utilizes the LPP produced X-rays more efficiently. One solution for collimating light from a point source is to place the point source at the focus of a paraboloidal reflector, thereby collecting and redirecting the light. Unfortunately, there are no known materials which in their naturally occurring form can be used as high efficiency reflectors in the X-ray wavelength region.
For nearly 20 years, researchers have been advancing a technology called synthetic multilayer reflectors' (ML). MLs are one-dimensional arrays of thin films of alternately high and low index of refraction. They constitute Bragg scatterers whose characteristic spacing, the d-spacing, is engineered to produce good reflectivity at near normal angles of incidence in the X-ray wavelength region. The Bragg condition is EQU sin.theta.=.lambda./2d EQ 1
where .theta. is the angle of grazing incidence, .lambda. is the wavelength, and d is the combined thickness of the two alternating thin film layers.
Fabrication of MLs is accomplished by using a two material sputtering system to deposit thin films on a substrate. For example, a constant d-spacing ML is fabricated using a flat substrate and repeatedly depositing alternating thin film layers of 20 A tungsten and 28 A carbon. The d-spacing for this ML is 48 A, the combined thickness of the alternating tungsten and carbon layers. Other thin film materials that are commonly alternated to produce MLs are molybdenum/silicon and rhodium/carbon.
A paraboloidal surface coated with a constant d-spacing ML will only behave as a good reflector and hence as a good collimator within the small range in which the Bragg condition is satisfied. If the point source is located at the focus of a paraboloidal reflector, the angle of incidence of the outgoing rays will vary as a function of axial position along the reflector. In order to achieve good reflection at all axial points, the d-spacing must vary with axial position according to Bragg's law (EQ 1).
For this reason attempts have been made to construct a paraboloidal collimator using a graded ML. Grading means that the d-spacing is varied, or graded, as a function of the axial position on the paraboloidal surface. The grading curve d(x) is chosen so that the Bragg condition is satisfied at every point for a single wavelength of X-rays from the point source.
This type of paraboloidal collimator will produce collimated light, but will not produce uniform intensity in a plane perpendicular to the collimated X-rays. The principal reason for this is that in the perpendicular plane, parameterized by the axial distance x, a length element dx subtends a solid angle which is itself a function of x. That is, d.OMEGA.=d.OMEGA.(x). For a paraboloid with constant reflectivity R, the collimated intensity in the perpendicular plane is ##EQU1## where .alpha. is the focal length of the paraboloid and I.sub.0 is the source intensity into 2.pi. steradians. Depending on the design parameters, the intensity variation across the perpendicular plane field may be greater than 100% of its minimum value. Therefore, the geometry of a paraboloidal collimator causes intensity variation of the collimated X-rays.
When using a collimator in a lithography system, the resist lies in a plane perpendicular to the collimated X-rays. The resist response or absorbed dose rate is dependent on the wavelength-dependent absorption of the resist and the intensity of the collimated X-rays. Therefore, either the geometry of a paraboloidal collimator or wavelength variation in the collimated X-rays can produce a non-uniform resist response.
Uniform resist response is required for VLSI lithography. In addition, due to the high cost of producing X-rays using a LPP, preserving the intensity of the collimated LPP X-rays improves the economic viability of collimated LPP X-ray technology for VLSI lithography. Therefore, there is a need for an X-ray collimator that provides a uniform resist response while also providing high intensity collimated X-rays.